The present invention relates to microfabricated devices, in particular microfluidics or nanofluidics devices which are treated to provide a non-wetting, non-absorbing coating thereon, as well as to processes for their production.
Microfabrication techniques have long been used in the electronics industries to produce items such as integrated circuit boards or printed circuit boards (PCBs) for increasingly miniaturised electronic devices. These techniques are finding application in other areas of technology.
Nanotechnology is a fast growing area of technology in which materials and devices are designed, synthesised and characterised on a nanoscale for a wide variety of applications, for example in microelectronics, semiconductors, optoelectronics, medicine/pharmaceutical, diagnostics, catalysis, filtration, energy storage, within the chemical or nuclear industries etc.
Materials and devices classified as nanotechnology devices are usually less than 100 nanometers in size. They are generally produced in one of two basic ways, the first of which involves the careful construction of the device, molecule by molecule to achieve the desired structure. The second method involves the gradual stripping or etching of material from pre-existing structures, and is largely based upon pre-existing microfabrication technology, such as that used in conventional semiconductor art.
Microfluidic or nanofluidic devices are miniaturized devices with chambers and tunnels for the containment and flow of fluids.
Microfluidic devices may be defined as having one or more channels with at least one dimension less than 1 mm, whilst nanofluidic devices will have generally smaller channels. With devices measured at the micrometer level and fluids measured in nanoliters and picoliters, microfluidics devices are widely used for example in biotechnology or biochemistry.
These devices can be used to handle a wide variety of liquids sample types. However, they are particularly useful in biochemical research or diagnostics in particular clinical diagnostics, where they may be used to handle liquids such as blood samples (including whole blood or fractions such as blood plasma), bacterial cell suspensions, protein or antibody solutions and other reagents including organic solvents, buffers and salts. Depending upon the nature and arrangement of the microfluidic device, it can be used in a wide range of analytical techniques and methods including for example, the measurement of molecular diffusion coefficients, fluid viscosity, pH, chemical binding coefficients and enzyme reaction kinetics. Other applications for microfluidic devices include capillary electrophoresis, isoelectric focusing, immunoassays, flow cytometry, sample injection of proteins for analysis via mass spectrometry, amplification of nucleic acids for example using amplification reactions such as the polymerase chain reaction (PCR), DNA and protein analysis, cell manipulation, cell separation, cell patterning and chemical gradient formation, high through-put screening, micro chemical manufacture, cell based testing of drug candidates, patient monitoring, proteomics and genomics, chemical microreactions, protein crystallisation, drug delivery, scale-up to manufacturing of drugs, security and defense.
The use of microfluidic devices in carrying out biomedical research and analysis has a number of significant advantages. First, because the volume of fluids within these channels is very small, usually several nanoliters, the amount of reagents and analytes used is quite small. This is especially significant for expensive reagents or where reagents are scarce, for example in some diagnostic applications or forensic DNA analysis.
The fabrications techniques used to construct microfluidic devices can be relatively inexpensive and are very amenable both to highly elaborate, multiplexed devices and also to mass production. In a manner similar to that for microelectronics, microfluidic technologies enable the fabrication of highly integrated devices for performing several different functions on the same substrate chip. These devices can therefore give rise to the so-called “lab-on-a-chip” devices, which can be used as portable clinical diagnostic devices for use for example in doctors' surgeries or hospitals or even at home as a point-of-care device, reducing the need for laboratory analysis procedures.
Microfluidic devices can be fabricated from a variety of materials, such as silicon, glass, metals or polymers or mixtures of these using a variety of microfabrication techniques. The selection of the particular technique depends to a large extent upon the nature of the substrate material. Depending upon the intended use, the substrate may be required to be quite rigid or stiff, or have a particular resistance to chemicals or temperature cycling to ensure any necessary dimensional stability.
For example, the manufacture may be carried out by laying down a photoresist (positive or negative) onto a substrate and in particular a silicon substrate. The photoresist is exposed to UV light through a high-resolution mask with the desired device patterns, so as to allow polymerisation to occur in the exposed areas. Then excess unpolymerized photoresist is washed off and the substrate is placed in a wet chemical etching bath that anisotropically etches it in locations not protected by photoresist. The result is a substrate such as a silicon wafer in which microchannels are etched. A coverslip such as a glass coverslip for instance, is used to fully enclose the channels and holes are drilled in the glass to allow access to the microchannels for the sample.
Deep reactive ion etching (DRIE) may be used as an alternative to this type of wet chemical etching which is particularly useful when straighter edges and a deeper etch depth is required.
Thermosetting or other curable polymers may also be used to prepare microfluidic devices, by moulding methods. A particular example of such a polymer is the silicone polymer, polydimethylsiloxane (PDMS) but others as are conventional in the art may be employed. The polymer in liquid form is poured over or into a mould (usually silicon or photoresist) and cured to cross-link the polymer. PDMA produces an optically clear, relatively flexible material that can be stacked onto other cured polymer slabs to form complex three-dimensional geometries.
Alternatively, polymers or plastics can be subject to hot embossing techniques so as to imprint suitable patterns into the surface of the plastics. Injection moulding may be used to create complex structures.
Some microfluidic devices are prepared from layered polymeric sheets. Outlines of the microfluidic device are cut in thin sheets of optically transparent plastics such as Mylar™ with a laser cutting tool such as a carbon dioxide laser. The layers are bonded together with a thin adhesive layer to produce three-dimensional structures.
All these techniques are useful and so microfluidics is showing great promise in a variety of applications as outlined above.
However the small volumes involved mean that the liquids are prone to surface effects, and in particular wetting or adsorption within the channels. The devices are generally less sensitive than bulk tests, and are prone to failure if insufficient liquid is able to pass along the channels. However the varying nature of the substrates used in these devices means that it is difficult to ensure that this does not happen.
Techniques which have been used to address this problem include sputtering TEFLON®-like coatings onto the devices or using fluorinated silanes in their construction. However, these techniques present further complications such as poor adhesion quality, lack of durability and ineffective control of film thickness.
Ionisation techniques or activation techniques, where reactive atoms or molecules such as ions or free radicals are generated and contacted with surface have been used to modify surfaces. Examples of such techniques include plasma processing (including plasma deposition and plasma activation), neutron activation, e-beam or thermal ionisation techniques. They have been quite widely used for the deposition of polymeric coatings onto a range of surfaces, and in particular onto fabric surfaces.
Plasma polymerisation in particular is recognised as being a clean, dry techniques that generates little waste compared to conventional wet chemical methods. Using this method, plasmas are generally generated from organic molecules, which are subjected to an electrical field. When this is done in the presence of a substrate, the radicals of the compound in the plasma react on the substrate to form a polymer film.
Conventional polymer synthesis tends to produce structures containing repeat units that bear a strong resemblance to the monomer species, whereas a polymer network generated using a plasma can be extremely complex due to extensive monomer fragmentation. The properties of the resultant coating can depend upon the nature of the substrate as well as the nature of the monomer used and conditions under which it is deposited.
WO03/082483 describes the deposition of non-uniform plasma polymeric surfaces onto devices so as to achieve certain specific technical effects such as the control of local wettability, adhesion and frictional/wear characteristics.
Plasma deposition of a uniform polymeric coating onto microfabricated devices and in particular microfluidic or nanofluidic devices in order to reduce wetting generally and increase reliability has not previously been described. It is not clear therefore whether coatings applied in this way would be effective at eliminating adsorption problems at this level.
The present inventors have found that by subjecting at least the surfaces of a microfabricated device which come into contact with a liquid during use to a ionisation or activation means such as a plasma which causes modification of the surface to impart non-wetting properties, the reliability and robustness of the microfabricated device may be significantly enhanced.